Influência de variáveis de sinterização na microestrutura de peças de PTFE moldadas por prensagem isostática

O politetrafluoretileno (PTFE) é utilizado em extensa gama de aplicações críticas devido à sua excelente resistência química e térmica, baixa energia superficial e propriedades tribológicas. Devido à sua elevada viscosidade no estado fundido, o PTFE não pode ser transformado pelos métodos tradicionais de extrusão e injeção, sendo o principal método de transformação deste polímero a prensagem sob temperatura ambiente, seguida de sinterização a temperaturas acima do ponto de fusão. O tempo de sinterização é definido de acordo com as dimensões da peça fabricada, podendo variar de poucas horas até vários dias para peças de grande volume. Avaliações sobre a influência dos extremos de tempo e temperatura de sinterização, atualmente adotados na indústria, sobre a microestrutura cristalina do PTFE, são escassas na literatura científica, sendo o principal objetivo deste estudo. Placas em PTFE foram sinterizadas variando-se a temperatura entre 360 ○C e 390 ○C e o tempo entre 10 e 10.000 min. Calorimetria exploratória diferencial (DSC), medidas de perda de massa e de densidade e microscopia eletrônica de varredura (MEV) foram utilizadas. Os resultados das medidas de perda de massa indicaram que a degradação do PTFE aumenta com o tempo e temperatura de sinterização. Análises das entalpias de fusão e medidas de densidade apontam indiretamente a redução na massa molar e aumento no grau de cristalinidade com o aumento do tempo e temperatura de sinterização. As análises em MEV possibilitaram a observação direta da microestrutura cristalina, indicando uma tendência de aumento da largura das lamelas com o tempo e temperatura de sinterização. Os resultados obtidos podem auxiliar no controle da microestrutura do PTFE durante o processamento, o que é bastante útil para a fabricação de peças em PTFE com desempenho otimizado

Influence of rotational speed on the microstructure and mechanical performance of friction-riveted thermosetting composite joints

Facing the actual demand for efficient joining technologies for multi-materials structures, Friction Riveting was shown to be an alternative joining technology for thermoset composite profiles in civil infrastructure. This process is based on plasticizing and deforming the tip of a rotating metallic rivet within a polymeric component through frictional heating. The feasibility of friction-riveted hybrid joints of Ti-6Al-4V/glass-fiber reinforced thermoset polyester was already demonstrated in a separate work. This paper complements this study by analyzing the rivet rotational speed effect on the process temperature, joint microstructure and the local and global mechanical properties of the joint. Joints were produced using two different levels of rotational speed: 9000 rpm and 10000 rpm (the other parameters were kept constant). The results showed process temperatures (655-765 °C) up to 96% higher than the onset decomposition temperature of the polyester matrix (370 °C); this led to severe degradation of the composite in the joint area. The increase in rotational speed, and therefore in heat generation, led to a statistically insignificant increase of the rivet penetration depth and the rivet diameter widening. However, the extension of the degraded composite area increased 47% which was responsible to deteriorate in 50% the joint tensile strength (from 4.0 ± 1.2 kN to 2.0 ± 0.7 kN). Moreover, the microhardness map of the joined rivet evidenced possible phase transformations in the alloy, favoring the material hardening by increasing in rotational speed. However, no correlations could be established between the changes in hardness and the joint tensile strength since the joints majority failure by full rivet pull-out. Thereby, for the improvement of friction-riveted Ti-6Al-4V/ glass-fiber reinforced thermoset polyester joints, the optimization of rotational speed is essential. This can guarantee the formation of efficient anchored joints and wider rivet tip deformation, concomitantly with the minimizing of the extension of the matrix degradation and finally leading to better tensile strength of the joints

Influence of the interlayer film thickness on the mechanical performance of AA2024-T3/CF-PPS hybrid joints produced by friction spot joining

Friction Spot Joining (FSpJ) is an innovative friction-based joining technique for metalpolymer hybrid structures. Friction spot joints of aluminum alloy 2024-T3 and carbon‑fiber reinforced poly(phenylene sulfide) composite laminate (CF-PPS) were produced with an additional PPS film interlayer. Two different film thicknesses were investigated in this study: 100 and 500 μm. Lap shear testing demonstrated that the joints produced with 100 μm film (2093 ± 180 N) were stronger than the joints with 500 μm (708 ± 69 N). Additionally, the fracture surface analysis revealed a larger bonding area for the joints with 100 μm film (53 ± 2 mm2) as compared to the joints with 500 μm film (40 ± 1 mm2). Considering the low thermal conductivity of PPS, the thinnest film is more likely to soften by the frictional heat during the joining process. Hence, the low viscosity of the molten PPS favors the wettability of the parts’ surface. Microstructural analyses proved that the metallic nub formation and the interdiffusion of PPS chains between film and composite matrix are also favored for thinner film use. Thus, superior adhesion between the partners is achieved. Therefore, it was concluded that the addition of the thinnest film interlayer leads to stronger joints

Viscometric characterization of PS/POSS hybrid nanocomposites

Nanocompósitos híbridos de poliestireno (PS) e poliedros oligoméricos silsesquioxanos (POSS) com diferentes composições e graus de hibridização foram obtidos por processamento reativo no estado fundido utilizando-se peróxido de dicumila (DCP) como iniciador, na presença ou não de estireno como agente de transferência de radical. Os materiais foram caracterizados viscosimetricamente por cromatografia de permeação em gel (GPC) usando detecção tripla por espalhamento de luz, viscosimetria e índice de refração. As amostras PS/POSS processadas com estireno apresentaram maiores valores de massa molar ponderal média (M) e menores valores de polidispersão (M /M), devido ao maior grau de conversão da reação de hibridização do PS-POSS (28-40%) e do menor grau de degradação (cisão) das cadeias do PS, quando comparadas com amostras PS/POSS processadas sem estireno nas quais o grau de conversão ficou em torno de 24-28%. Para os sistemas PS e PS/POSS em solução com THF, os parâmetros da equação de Mark-Houwink-Sakurada (MHS), α Ξ 0,7 e log K Ξ –3,5 a –3,9 e os valores dos parâmetros de interação polímero-solvente, χij Ξ 0,49, não apresentaram diferenças significativas com relação aos tamanhos moleculares. Por outro lado, essas diferenças de tamanhos moleculares foram caracterizadas por uma função cumulativa da fração mássica de cadeias em função da distância média quadrática entre pontas de cadeia (0 1/2 ).Polystyrene (PS) and polyhedral oligomeric silsesquioxanes (POSS) hybrid nanocomposites with different compositions were obtained by reactive melt processing using dicumyl peroxide (DCP) as initiator in the presence or absence of styrene as radical transfer agent. The materials were characterized by viscosimetry by means of gel permeation chromatography (GPC) using triple-detector: light scattering, viscometer and refractive index. PS/POSS samples processed with styrene showed higher weight average molecular weights (Mw) and lower polydispersity indexes (Mw/Mn), as a result of higher PS-POSS conversion (28-40%) and lower PS degradation, as compared to the PS/POSS samples processed without styrene in which the degree of conversion was lower (24-28%). For the PS/POSS solutions in THF, the parameters of the Mark-Houwink-Sakurada equation, α Ξ 0.7 and log K Ξ –3.5 to –3.9, and the values of polymer-solvent interaction parameter, χij Ξ 0.49, were not changed with respect to changes in molecular size. On the other hand, these changes were characterized by a cumulative function of the mass fraction of chains as a function of the root mean square end to end distance (0 1/2 )

Friction riveting of aluminium alloy 6056 T6 and polyamide 6: role of the rotation speed on the formation of the anchoring zone and mechanical performance

Hybrid metal-polymer structures are an alternative solution to reduce weight and fuel consumption in the transportation industry in order to minimize the emission of noxious gases in regard to the greenhouse effect. Friction Riveting is a relatively new technique for joining metal-polymer hybrid structures. The process relies on the generation of frictional heat between the components causing the plastic deformation of the metallic rivet and its anchoring in the polymer component. This study evaluated the technical feasibility of friction-riveted AA 6056 T6 and PA6 joints, and the influence of the rotational speed (RS) on the maximum process temperature and on the mechanical performance of the joints. The maximum temperature reached increased with the rotational speed, from 291 ± 6 °C at 10000 rev/min to 375 ± 5 °C at 15000 rev/min. The use of greater rotational speeds induced the plastic deformation of the tip of the metallic rivet during the frictional phase. This led to mechanically stronger joints due to the larger anchoring of the metallic rivet within the polymeric plate. The AA 6056 T6-PA6 joints had good tensile strength, achieving 85% of the metallic rivet’s tensile strength. Therefore, the feasibility of friction-riveted AA 6056 T6-PA6 joints was proven. Furthermore, it was shown that the rotational speed influences directly the rivet anchoring and thus the tensile strength of the joints